1. Field of the Invention
This invention relates to a spread-spectrum communication system in which, when information is transmitted by wire or wirelessly, the information is converted into a signal having a bandwidth much greater than that of the information band.
2. Description of the Prior Art
A spread-spectrum communication system is one in which information data is transmitted as a signal whose bandwidth is made much larger than that of the data. Broadly classified, two methods are available for achieving such communication. One method is a so-called direct-sequence (DS) method, in which the transmitting side multiplies a digitized baseband signal by a spread spectrum code such as a high-speed pseudorandom noise code to generate a baseband signal having a bandwidth much greater than that of the original data. The generated signal is modulated as by phase-shift keying (PSK) or frequency-shift keying (FSK) to be converted into a radio-frequency (RF) signal before being transmitted. On the receiving side, a spread-spectrum code the same as that on the transmitting side is used to perform reverse spreading, which is for establishing correlation with the received signal. Thus, demodulation is performed to obtain the original data.
The second method is referred to as so-called frequency hopping (FH), in which the transmitting side modulates a carrier wave by a baseband signal and transmits a signal by periodically changing frequency, in accordance with a spread-spectrum code, every data bit or at a time interval which is a whole-number fraction or a whole-number multiple thereof. On the receiving side, demodulation is performed to obtain the original data by carrying out reverse spreading. This is accomplished by performing a correlating operation, in which the local carrier wave on the receiving side is brought in tune with the transmitting side by using a spread-spectrum code the same as that on the transmitting side.
In order to achieve correct correlation on the receiving side in these systems, it is necessary for the spread-spectrum code on the transmitting side and that on the receiving side to be synchronized accurately. In the prior art, synchronizing circuits for achieving this synchronization employ a so-called sliding correlation loop.
FIG. 4 is a diagram illustrating a sliding correlation loop for the DS method. As shown in FIG. 4, a received spread-spectrum signal enters a mixer 401, which multiplies this input by a spread-spectrum code sequence generated by a spread-spectrum code-sequence generator 406. The output of the mixer 401 enters a band-pass filter (BPF) 402 having a bandwidth corresponding to that of the original data prior to spreading. The filtered output of the BPF 402 enters a detector circuit 403, which subjects this input signal to envelope detection. The output of the detector circuit 403 enters a low-pass filter (LPF) 404 to be smoothened thereby.
If autocorrelation is achieved, the output of the mixer 401 will be a signal resulting from reverse spreading of the received spread-spectrum signal. This output signal passes through the BPF 402 and has its envelope detected by the detector circuit 403. The output signal of the detector circuit 403 enters the LPF 404, which proceeds to smoothen the signal to obtain a DC level.
If autocorrelation is not achieved, a signal which is the result of reverse spreading of the received spread-spectrum signal is not obtained at the output of mixer 401, and therefore almost all of the power of the received spread-spectrum signal is blocked by the BPF 402. The output of the BPF 402 is subjected to envelope detection by the detector circuit 403, whose output signal is smoothened by the LPF 404. As a result, the DC level obtained is sufficiently small in comparison with that obtained at autocorrelation.
The DC-level output of the LPF 404 is supplied to a voltage-controlled oscillator (VCO) 405. Since the output of the LPF 404 has a sufficiently small DC level when autocorrelation is not achieved, the VCO 405 in such case produces an output whose frequency is somewhat different from that of the spread-spectrum code contained in the received spread-spectrum signal. The VCO 405 supplies this output to the spread-spectrum code-sequence generator 406 as a clock signal. In this case, since the speed of the clock of the spread. spectrum code generated by the spread-spectrum code. sequence generator 406 is offset slightly from the clock speed of the received spread-spectrum signal, the phases of the two signals gradually become displaced from each other. As a result, by the time the two phases shift by an amount equivalent to one period of the spread. spectrum code, coincidence is achieved between the spread-spectrum code in the received spread-spectrum signal and the spread spectrum generated by the code-sequence generator 406, and therefore autocorrelation is obtained. When this occurs, the DC output level of the LPF 404 rises, so that the oscillation frequency of the VCO 405 changes to a frequency synchronized to the received spread-spectrum signal, and the system stabilizes at this frequency. Thus, synchronization is achieved between the received spread-spectrum code and the spread-spectrum code generated by the spread-spectrum code sequence generator 406. The time needed to obtain synchronization with this method generally is very long since the phase of the received spread-spectrum code shifts only in gradual fashion.
FIG. 5 illustrates a sliding correlation loop for the FH method. The construction of the loop in FIG. 5 is similar to that of FIG. 4 except that a frequency synthesizer 507 is added.
In FIG. 5, the synthesizer 507 changes the frequency of the output signal in accordance with the pseudorandom noise signal which enters from a spread-spectrum code-sequence generator 506. A mixer 501 multiplies the output signal of the synthesizer 507 and the received spread-spectrum signal together. If the spread-spectrum code on the receiving side is in synchronism with that on the transmitting side, the signal obtained at the output of the mixer 501 will be a signal whose frequency band is the same as that of the signal prior to spreading. Operations performed by a BPF 502, detector circuit 503, LPF 504, VCO 505 and the code-sequence generator 506 are the same as in the DS method.
Thus, in conventional spread-spectrum communication systems, the spread-spectrum code is changed along a time axis. This means that circuitry for achieving spread-spectrum code synchronization is required on the receiving side, and that the time needed for achieving such synchronization by this circuitry is very long. These are some of the drawbacks encountered in the prior art.